1. Field of the Invention
This invention relates to pressing or transferring holograms and/or diffraction images into metallic surfaces such as metal cans.
2. Description of the Prior Art
It is well known to manufacture a variety of products having holograms or diffraction images pressed or embossed into them. Such products include credit cards, license plates, consumer packaging, pictures, tickets and the like. See, for example, U.S. Pat. Nos. 4,368,979; 4,684,795; 4,856,857; 4,971,646; 4,999,075; and 5,267,753. Holograms generally mean a diffraction pattern which produces a three-dimensional image. Other diffraction images modify the behavior of light that strikes an object bearing the image without creating a three-dimensional appearance. The term "hologram" is used herein in the broad sense to include diffraction patterns or images as well as three-dimensional holograms and other complex images having different planes of diffraction. The manufacture of holograms, shims or plates is well known in the art as disclosed in U.S. Pat. Nos. 3,733,258; 3,875,026; 3,944,420; and 5,059,499, among others.
Most holograms on credit cards and the like are embossed into a plastic sheet material such as Mylar.RTM. or other polyester which has a thin underlayer of reflective material such as aluminum to produce a reflective hologram. The aluminum is typically vapor deposited onto the substrate. Holograms can also be impressed into thermoplastic layers on durable substrates as disclosed in U.S. Pat. No. 4,913,858. The coated sheet is heated to soften the coating which is decorated using an embossing member to provide a diffraction pattern or hologram.
It is further known to create embossing tools or roller dies bearing holographic patterns which are impressed into aluminum foil, translucent plastics and other materials, as is disclosed in U.S. Pat. No. 5,521,030. That patent discloses a process in which the holographic pattern is etched directly into a durable surface on the tool or die. The patent states that large quantities of holograms can be made from a single die made of high quality steel, chrome, or the like that is used in the roller die. The patent further states that the die may be a flat plate mounted on a substrate, an inside surface of an injection mold, a mandrel, a casting mold, or other surface used for transferring a relief pattern.
It is also known to emboss holograms directly into the outer surface of sheet metal such as aluminum foil and aluminum beverage cans as is disclosed in U.S. Pat. Nos. 4,773,718 and 4,725,111. Those patents explain that the temperature of the aluminum is important for optimizing the embossing process and that the nickel master plate must be many times less deformable at the embossing temperature than is the material being embossed. The patents describe a process in which the aluminum foil and aluminum cans are heated to reduce the yield strength of the metal. The yield strength of the nickel master is relatively constant over the temperature range in which the yield strength of the aluminum in the foil and cans is dramatically reduced. The patents disclose embossing apparatus having a metal embossing plate carried on the outside surface of a wheel which is pressed against a can mounted on a mandrel. The embossing plate is heated by a heater that may be disposed within the wheel. The machine has a plurality of mandrels for cans which are sequentially embossed. The mandrels are preferably cooled. The process disclosed by these patents has not been used commercially despite the desirability of enhancing the decoration of cans.
An improved low cost technique is desired for embossing holograms into metal cans and other objects. Many billions of aluminum beer and beverage cans are made and sold annually, and improved decoration of such cans would improve their marketability. Aluminum cans are made from hard temper aluminum alloys, and the sidewalls of the cans are quite thin such as in the range of about 0.0038 to 0.0042 inch. The cans are typically filled with carbonated beverages which produce pressures of 90-110 psi or more within the cans. The cans are commonly closed with a lid that is secured on the cans by a double seamer that applies a top load of about 100 to 130 pounds on the can during the seaming operation. It is important that the temper and strength of aluminum cans not be unduly reduced during processing as can happen if the cans are heated.
A technique for embossing holograms on cans is needed which adds little to the cost of the cans and which produces consistently high quality images. It is desirable for a manufacturing system to be capable of embossing holograms on approximately 2.5 million cans per day in order to be commercially practicable. Beverage companies and bottlers demand high quality cans and will not accept noticeable variations in the decoration on their cans. Moreover, the commercial can manufacturing and filling processes operate at high speeds of 600 to 2000 cans per minute, and can decoration processes must be statistically in control and capable at such speeds.